Connection Of Prestressing Sheath Sections Of A Structure Having A Series Of Precast Elements

ABSTRACT

The prestressed building structure comprises an assembly of precast elements separated by gaps occupied by an interface product, one or more prestressing sheaths having sections respectively incorporated into the precast elements, and a prestressing tendon tensioned inside the sheath. The sheath sections are respectively fitted with first and second end pieces opening out on the facing surfaces of the two precast elements. The first end piece has a flared opening. An elastic connecting sleeve is connected in a sealed manner to the second end piece. The sleeve presses against the first end piece, which compresses it longitudinally to ensure a seal between the inside of the sheath sections and the gap separating the adjacent surfaces of the two elements.

BACKGROUND OF THE INVENTION

The present invention relates to the assembly of precast constructionelements for building prestressed structures.

It applies in particular, but not exclusively, to the decks ofcantilevered bridges built using precast concrete elements (segments).These structures are frequently subjected to longitudinal prestressingusing prestressing tendons threaded inside sheaths embedded in theconcrete of several successive elements.

Carrying out such prestressing is a difficult operation. The positioningof the sheath sections in the elements must be very accurate so that theprestressing tendons can be threaded without difficulty.

Moreover, the sealing of the sheath at the interfaces between theelements must be ensured. This sealing is necessary to ensure thedurability of the prestressing, which is subject to the risk ofinfiltrations at the joint between the elements.

When the joint is made using the so-called “wet joint” method, aninterface product such as concrete or mortar fills the gap between twoadjacent elements. In this case, the seal also meets the need to preventthe interface product, or certain components of it, from entering thesheaths when it is placed between the elements, and then hindering theinsertion of the tendons.

Furthermore, the sheaths are often injected with a filler (cement grout,grease, wax, resin, etc.) serving in particular to protect the tendonsagainst corrosion. This product must not escape from the sheath duringinjection. Some areas of the structure can have a relatively highdensity of sheaths, and there is no guarantee that the interface productwill produce a seal between these sheaths. As a result, there is aserious risk that grout injected under pressure into a sheath willinfiltrate into one or more neighbouring sheaths, in which injectionthen becomes very difficult or even impossible.

FR-A-2 596 439 describes a linking device between sections ofprestressing sheath, comprising a cylindrical sleeve engaged between theopenings of two adjacent sections to ensure the continuity of thesheath, and an elastic seal surrounding the cylindrical sleeve to ensuresealing and compensate for the positioning variations and dimensionaldeviations of the blocks, which are assembled against each other.

WO 99/043910 describes an improvement of the construction methods inwhich the elements are matched and then assembled in contact with eachother. The matched elements are cast using the so-called “match cast”technique in which the front surface of an n^(th) element defines therear side of the mould used to shape the (n+1)^(th) element of theseries. When each element is cast, sleeves are arranged at the ends ofthe sheath sections that they contain. The complementary surfaces of thematched elements are then pressed against each other so that the sheathsections are arranged running on from each other to form completesheaths. Positioning connectors are engaged in the sleeves to connectthe adjacent sheath sections in a sealed manner.

This last technique is well-suited to elements cast using the “matchcast” technique, which ensures accurate mutual positioning of theadjacent sections of a prestressing sheath. However, its implementationcan be difficult if a “wet joint” method is used. In this case, theelements are often factory cast with significant dimensional tolerances,and the gap between the adjacent surfaces of two successive elements canbe several centimetres.

SUMMARY OF THE INVENTION

The present document relates to an assembly technique for precastelements that provides an answer to the issue of sealing the sheathswhen a “wet joint” method is used.

It sets out a construction method for a prestressed structure having aseries of precast elements. This method comprises the steps of:obtaining two successive elements in the series, each of the twoelements incorporating at least one prestressing sheath section and anend piece linked to said sheath section and opening out on a surface ofsaid element, the end piece incorporated into one of the two elementshaving a flared opening; connecting in a sealed manner an elasticconnecting sleeve to the end piece incorporated into the other element;arranging the two successive elements relative to each other,maintaining a gap between two adjacent surfaces of the elements, theconnecting sleeve being engaged in said flared opening and compressedlongitudinally by the bringing together of the elements, the compressionof the connecting sleeve ensuring a seal between the inside of thesheath sections and the gap between the adjacent surfaces of theelements; and placing an interface product in the gap between theadjacent surfaces of the elements.

The elastic connecting sleeve separates in a sealed manner the inside ofthe prestressing sheath from the gap between the two elements, whichmust be filled with concrete or another interface product. Thearrangement allows for a seal to be produced without accessing theconnecting area, which is prevented by the narrowness of the gap betweenthe elements. However, this gap has a significant thickness and itsdimensions are not accurately guaranteed given the manufacturingtolerances of the elements and possible inaccuracies on assembly. Therecan also be misalignments between the sheath sections and end piecesrelative to their theoretical positioning in the elements. The gradualflaring of at least one of the end pieces and the elasticity of theconnecting sleeves that extend between the two end pieces allows forthem to be deformed in such a way as to compensate for the variousdeviations and inaccuracies linked to the manufacturing and assembly ofthe elements.

The gap between the adjacent surfaces of the elements can for examplehave a thickness of between 3 and 6 centimetres. A typical order ofmagnitude for the longitudinal compression capacity of the connectingsleeve is a capacity greater than one centimetre. Moreover, a typicalorder of magnitude for the misalignment between the two end piecespermitted by the connecting sleeve is in an angular range greater thanone degree.

Another aspect of the invention relates to a building structurecomprising: an assembly of at least two precast elements having tworespective facing surfaces separated by a gap occupied by an interfaceproduct; at least one prestressing sheath having two sectionsrespectively incorporated into the precast elements; and a prestressingtendon tensioned inside the sheath. The sheath sections are respectivelyfitted with first and second end pieces opening out on the facingsurfaces of the two precast elements, the first end piece having aflared opening. An elastic connecting sleeve connected in a sealedmanner to the second end piece is pressed against the first end piece,which compresses it longitudinally to provide a seal between the insideof the sheath sections and the gap separating said surfaces.

A further aspect of the invention relates to a connection system forprestressing sheath sections, comprising: first and second end pieces,each having a rear side capable of being connected to a sheath sectionincorporated into a respective precast construction element and a frontside to open out on a surface of said element, the front side of thefirst end piece having a flared opening; and an elastic connectingsleeve having one side capable of being connected in a sealed manner tothe second end piece and an opposite side capable of cooperating withthe first end piece, the connecting sleeve having a longitudinalcompression capacity and a transverse deformation capacity in order tobe compressed when said surfaces of the elements are brought together,whilst permitting an offset and misalignment between the two end pieces,the compression being capable of providing a seal between the inside ofthe sheath sections and a gap separating said surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following description of non-limitative embodimentexamples, with reference to the attached drawings.

FIG. 1 is a perspective view of a precast segment to which the methodaccording to the invention can be applied.

FIG. 2 is a diagram showing the juxtaposition of two adjacent segmentsequipped with prestressing sheath sections and forming part of a seriesof precast elements.

FIG. 3 is a block diagram of a connection system according to oneembodiment of the invention.

FIG. 4 is a diagram of a female end piece of the system in thisembodiment.

FIG. 5 is a diagram showing a possible arrangement of a prestressingsheath section near the end surface of an element.

FIGS. 6 and 7 are respectively profile and front diagrammatic views of avariant embodiment of a female end piece of the connection system.

FIG. 8 is a longitudinal cross-sectional view of another embodiment ofthe connection system according to the invention.

DESCRIPTION OF EMBODIMENTS

The invention is described below in its non-limitative application tothe cantilever construction of a precast segment bridge.

Such a segment 1 is shown in FIG. 1. The element 1 has the general formof a caisson delimited at the bottom by a base 2, laterally by twosymmetrically sloping walls 3, and at the top by a deck 4 extendedlaterally beyond the walls 3 to define the width of the bridge.

In the longitudinal direction, the element 1 is delimited bysubstantially parallel rear 7 and front 6 surfaces. The rear surface 7is intended to face the front surface, of complementary shape, of theprevious element installed on the structure under construction (for thefirst element installed on a pier of the bridge, the complementarysurface belongs to the pier). Similarly, the front surface 6 of theelement 1 is intended to face the rear surface of the next element to beinstalled. The complementary shaped surfaces of the adjacent elementscan possibly be provided with bosses 8 facilitating the relativepositioning of the elements when they are brought together.

The element 1 (or 1A, 1B in FIG. 2) comprises a number of longitudinalsheath sections 10 (10A, 10B in FIG. 2) intended to receive prestressingtendons 15. The prestressing tendons 15 are anchored onto the structureat their ends by means of appropriate anchors. Some of these anchors 11can possibly be arranged on sheaves 12 provided inside the caisson shapeof the element. The sheath sections 10 open out on the rear surface 7and/or the front surface 6 of the element. The continuity and sealing ofeach prestressing sheath 10 must be ensured at the interfaces betweenthe elements. To this end, a connection system is used, embodiments ofwhich are described below with reference to FIGS. 3 to 8.

After the positioning of an element 1B with a connection systeminstalled at the joints of the sheath sections 10A, 10B, an interfaceproduct 16, which will generally be concrete, is injected into the gapbetween the element 1B and the previous element 1A in the series. Thisgap typically has a thickness of between 3 and 6 centimetres. Thesealing of the sheath is important to prevent components of theinterface concrete 16 entering the sheath 10, which would hinder thesubsequent threading of the tendons 15.

Once the interface concrete has set, the next element is assembled. Ifone (or several) prestressing sheaths 10 has its (their) last section inthe element that has just been installed, the threading, anchoring andtensioning of a prestressing tendon 15 in this sheath can take place,possibly after having checked the seal using a pneumatic device.Threading can be carried out using conventional techniques. Aftertensioning, filler, generally cement grout, is injected into the sheath10 to protect the metal of the tendon 15 against corrosion. The sealingof the sheath is important to prevent grout injected in a fluid statefrom escaping at the interfaces between the elements.

The successive elements 1, 1A, 1B of the series are prefabricated fromcast concrete. In the embodiment shown in FIG. 3, the rear surface 7 ofelement 1B is facing the front surface 6 of the previous element 1A inthe series. At the interface, the sheath sections 10A, 10B embedded inthe concrete of elements 1A, 1B are respectively provided with two endpieces 20A, 20B also incorporated into the concrete of the element andmade for example from a rigid plastic material. In the example shown, amale end piece 20A has its rear side connected to the sheath section 10Aincorporated into the element 1A already in place on the structure,whilst a female end piece 20B has its rear side connected to the sheathsection 10B of the new element 1B on its rear surface 7. The end pieces20A, 20B are connected to the sheath sections 10A, 10B in a sealedmanner, and are placed in the mould used to produce the elements 1A, 1B.In general, the end pieces 20A, 20B do not extend beyond the end surface6, 7 of the element, for reasons of ease of casting. They can bepositioned in the mould using studs positioned at the appropriate placeson the inside surfaces of the walls of the moulds. After form removal,the front sides of the end pieces 20A, 20B open out on the surfaces ofthe elements, which will be placed facing each other when the bridgedeck is assembled.

In addition to the end pieces 20A, 20B, the connection system shown inFIG. 3 comprises an elastic connecting sheath 21 made from an elastomermaterial. To enhance the elasticity of the sleeve 21 and itsdeformability in both an axial direction and transverse to the directionX of the sheath at the interface between the elements, the sleeve can beshaped like a bellows, as shown in FIG. 3.

The sleeve 21 is connected in a sealed manner to the male end piece 20Aof the connection system. This connection is for example achieved byclipping or by screwing the rear side of the sleeve 21 in the male endpiece 20A. It takes place after the forms have been removed from theelement 1A. The front side of the connecting sleeve 21 cooperates withthe female end piece 20B of the facing element. The sleeve 21 and theend pieces 20A, 20B are sized so that the sleeve 21 is compressedaxially when the two elements 1A, 1B are brought together on assembly.

The female end piece 20B has an opening 22 that flares gradually asshown in FIGS. 3 and 4. This flare 22 facilitates the insertion of theconnection sleeve 21 without it being necessary to manipulate it whenthe elements 1A, 1B are brought together.

The elasticity of the sleeve 21 allows for tolerances to be permitted inthe accuracy of the production of the concrete elements 1A, 1B, whichtolerances are usually several centimetres. The sleeve 21 shouldtherefore have a longitudinal compression capacity greater than 1 cm.

Furthermore, it is very difficult to accurately guarantee thepositioning of the sheath sections 10A, 10B parallel to the surfaces 6,7 of the elements, as well as their orientation relative to thesesurfaces. The capacity of the sleeve 21 to deform transversely at thejoint plane between the elements 1A, 1B also allows for theseinaccuracies to be absorbed. The misalignment between the end pieces20A, 20B of the sheath sections that the sleeve 21 can compensate for isgreater than 1 degree and can even be around 10 degrees or more.

The gradual flaring of the opening of the female end piece 20B can be offrusto-conical shape, as shown in FIG. 4, with a half-cone angle αsufficient to facilitate the approach of the elastic connecting sleeve21. The flare 22 allows for the end of the connecting sleeve 21 to beconveyed to a recess 23 provided at the bottom of the female end piecewhen the two elements are brought together. The front end of the sleeve21 can be shaped so that it nests firmly in the recess 23 in order toensure, by clipping, a sealed connection under the action of the returnforce exerted due to the elasticity of the compressed sleeve. The flare22 can also contribute to deforming the sleeve 21 if the two sheathsections are not exactly aligned. For a frusto-conical flare 22 oflength L, the cone must have a sufficient opening at its base for thesleeve 21 to enter fully into the cone during the bringing together ofthe two elements 1A, 1B. The flare 22 must then compensate for: theeffect of any local gradient β of the sheath relative to the joint planebetween the elements 1A, 1B (see FIG. 5); the effect of the positioninginaccuracy of the two end pieces 10A, 10B in relation to each other(offset Δ parallel to the joint plane); the fact that the sleeve 21 isunfolded, i.e. not compressed, by a length D on bringing together.

Under these conditions, the minimum opening angle α of the coneverifies:

$\begin{matrix}{{\tan \; \alpha} \geq {{\tan \; \beta} + \frac{\Delta}{L \times \cos \; \beta} + \frac{D \times \tan \; \beta}{L}}} & (1)\end{matrix}$

Moreover, the opening of the cone can facilitate the sliding of thesleeve 21 towards the recess 23 despite the friction of the sleeve onthe female end piece 20B, whatever the mutual positioning defect of theend pieces. If the friction is defined by a cone with a half cone angleφ, another condition on the maximum opening of the frusto-conical flare22 is:

α<90°−β−φ

The frusto-conical shape with a circular cone for the flare 22 has theadvantage of being simple to produce. It also allows for the avoidanceof any ambiguity in the direction of placing the female end piece 20B inthe formwork, and therefore of any risk of error.

In certain cases however, the opening of the cone at the end surface 6,7 of the element can have relatively large dimensions, which can beproblematic, particularly when several neighbouring prestressing sheathshave to cross the gap between the two elements. Moreover, if the sheathsare embedded in a relatively narrow concrete part, such as a segmentweb, the width of the cone can become significant relative to the totalwidth of the part and lead to a weakening of the structure.

In the minimum angle condition (1), it can be observed that only theterm

$\frac{\Delta \;}{L \times \cos \; \beta}$

relating to the positioning tolerance of the end pieces relative to eachother is omnidirectional. The other two terms relating to the gradientof the sheath and the extension of the sleeve only operate in an apriori known direction, namely the direction of minimum angle betweenthe sheath section 10B and the joint plane. This direction of minimumangle is the direction in which the angle β is shown in FIG. 5.

Under these conditions, it can be prudent to provide an anisotropicflare of the female end piece, as shown in FIGS. 6 and 7. In thisembodiment, the front side of the female end piece 30B has one halfprovided with a frusto-conical circular flare (lower part of FIGS. 6 and7), with a half-cone angle α′ of the order of

${Arc}\mspace{11mu} {\tan \left( \frac{\Delta}{L \times \cos \; \beta_{\max}} \right)}$

where β_(max) is the maximum gradient of the sheath relative to thejoint plane. The other half of the front side of the female end piece30B (upper part of FIGS. 6 and 7) has a flare in the shape of a conewith an elliptical base, the half cone angle α of the cone on the majoraxis of the ellipse verifying condition (1), α then being of the orderof

${Arc}\mspace{11mu} {{\tan \left( {{\tan \; \beta_{\max}} + \frac{\Delta}{L \times \cos \; \beta_{\max}} + \frac{D \times \tan \; \beta_{\max}}{L}} \right)}.}$

For the assembly of such a female end piece 30B on its sheath section10B in the mould for the concrete element, the end piece is oriented sothat it presents its maximum flare, that is, the major axis of theelliptical shape, in the direction of minimum angle between the sheathsection and the joint plane.

Under these conditions, the performance of the connection system can beoptimum while limiting the extension of the end piece 30B on the surfaceof the element 1B in directions other than the direction in which it isgenuinely necessary.

The invention is not limited to the embodiments described above. Inparticular, the female end piece is not necessarily incorporated intothe new element that is being assembled: it can also be on thepreviously installed element. In another embodiment, the connectingsleeve 21 can be in one piece with the male end piece connected to thesheath section of one of the elements.

In yet another embodiment, such as that shown in FIG. 8, the end pieces40A, 40B incorporated into the two adjacent precast elements 1A, 1B aremade up of identical parts, which allows for their production cost to beminimised and avoids confusion during the casting of the concreteelements. In this example, each end piece 40A, 40B has an internallythreaded recess 43 capable of receiving in a sealed manner the threadedrear side of the elastic connecting sleeve 41. Beyond this recess 43,the end piece 40A, 40B ends in a frusto-conical flare 42 as previouslydescribed. Still in the example in FIG. 8, the elastic connecting sleeve41 has a generally M-shaped profile forming a bellows that permits bothlongitudinal compression and transverse offset. The arm of the M locatedon the front side of the sleeve 41 presses, when the elements arebrought together, against the flared opening 42 of the end piece 40Aincorporated into the other element. The sealing results from thecontact area between the front part of the sleeve and the frusto-conicalopening 42. In FIG. 8, the sleeve 41 is shown at rest by dashed lines,and in its compressed position by solid lines. It will be noted that thecompression of the sleeve gives rise to almost no encroachment on theinner section of the sheath, where the prestressing tendons will bethreaded.

1. A method of building a prestressed structure having a series ofprecast elements, the method comprising: obtaining two successiveelements in the series, each of the two elements incorporating at leastone prestressing sheath section and an end piece connected to saidsheath section and opening out on a surface of said element, the endpiece incorporated into one of the two elements having a flared opening;connecting in a sealed manner an elastic connecting sleeve to the endpiece incorporated into the other element; arranging the two successiveelements relative to each other, maintaining a gap between two adjacentsurfaces of the elements, the connecting sleeve being engaged in saidflared opening and compressed longitudinally by the bringing together ofthe elements, the compression of the connecting sleeve ensuring a sealbetween the inside of the sheath sections and the gap between theadjacent surfaces of the elements; and placing an interface product inthe gap between the adjacent surfaces of the elements.
 2. The method ofclaim 1, wherein the connecting sleeve has a longitudinal compressioncapacity greater than one centimetre.
 3. The method of claim 1, whereinthe connecting sleeve has a capacity to deform transversely to the jointplane between the elements.
 4. The method of claim 3, wherein theconnecting sleeve permits a misalignment between the two end pieces inan angular range greater than one degree.
 5. The method of claim 1,wherein the gap between the adjacent surfaces of the elements has athickness of between 3 and 6 centimetres.
 6. The method of claim 1,wherein said flared opening has a frusto-conical shape.
 7. The method ofclaim 1, wherein said flared opening has an anisotropic flare.
 8. Themethod of claim 7, wherein the sheath sections are sloped relative to ajoint plane between the two elements, and wherein the flared opening ofsaid end piece incorporated into one of the two elements has a maximumflare in a direction that is aligned on the direction of minimum anglebetween the sheath section incorporated into said element and the jointplane.
 9. A building structure, comprising: an assembly of at least twoprecast elements having two respective facing surfaces separated by agap occupied by an interface product; at least one prestressing sheathhaving two sections, respectively incorporated into said precastelements; and a prestressing tendon tensioned inside the sheath, whereinsaid sheath sections are respectively fitted with first and second endpieces opening out on the facing surfaces of the two precast elements,the first end piece having a flared opening, and wherein an elasticconnecting sleeve connected in a sealed manner to the second end pieceis pressed against the first end piece, which compresses itlongitudinally to ensure a seal between the inside of the sheathsections and the gap separating said surfaces.
 10. A prestressing sheathsection connection system, comprising: first and second end pieces, eachhaving a rear side capable of being connected to a sheath sectionincorporated into a respective precast construction element and a frontside to open out on a surface of said element, the front side of thefirst end piece having a flared opening; and an elastic connectingsleeve having one side capable of being connected in a sealed manner tothe second end piece and an opposite side capable of cooperating withthe first end piece, the connecting sleeve having a longitudinalcompression capacity and a transverse deformation capacity in order tobe compressed when said surfaces of the elements are brought together,whilst permitting an offset and misalignment between the two end pieces,the compression being capable of providing a seal between the inside ofthe sheath sections and a gap separating said surfaces.
 11. The systemof claim 10, wherein the connecting sleeve has a longitudinalcompression capacity greater than one centimetre.
 12. The system ofclaim 10, wherein the connecting sleeve permits a misalignment betweenthe two end pieces in an angular range greater than one degree.
 13. Thesystem of claim 10, wherein the first and second end pieces are twoparts of the same shape.
 14. The system of claim 10, wherein theconnecting sleeve is made in one piece with the second end piece. 15.The system of claim 10, wherein the connecting sleeve is shaped like abellows.
 16. The system of claim 10, wherein the first end piece has arecess to receive the front end of the connecting sleeve when saidsurfaces of the elements are brought together in order to ensure asealed connection between the first end piece and the connecting sleeveunder the action of a return force exerted due to the elasticity of thecompressed sleeve.
 17. The system of claim 16, wherein the recess andthe front end of the connecting sleeve are shaped in order to formbetween them a clipped connection when said surfaces of the elements arebrought together.
 18. The system of claim 10, wherein said flaredopening of the first end piece has an anisotropic flare.